The present invention relates to a multipass wiredrawing machine provided with a device for adjusting and controlling the tension of the wire being processed, particularly adapted for drawing metal wires and the like.
In particular, the invention relates to multiple soap and wet wiredrawing machines.
The term "drawing" designates a kind of plastic cold working which consists in forcing the material, by pulling it, through the shaped hole of a die. The arrangement in sequence of a plurality of dies with holes having decreasing diameters allows to reduce the original diameter to the intended size.
It is known that in order to draw metal wires the wire passes through a die where its cross-section is reduced by plastic deformation.
Traction is applied by a pulling reel arranged downstream of the die which, by turning, winds on itself a certain number of turns of the wire.
In the case of multipass wiredrawing machines, the operation is repeated several times in succession by guiding the wire; accordingly, a plurality of dies are provided which are arranged in sequence, each die having a corresponding reel arranged downstream of it.
The wire is thus guided from one reel to the next die with a consequent reduction in diameter between one transfer and the next.
The drawing action must be applied so that the wire being processed, by generating friction on the pulling reel actuated by an electric motor, allows said reel to pull it (with a capstan effect), wind it and then guide it to the next reel.
Since drawing is performed without removing material, i.e., with a constant volume, the ratio between the cross-section and the speed of the wire becomes particularly important and must be kept constant for each pass through the various dies and respective reels.
Accordingly, at every reduction in the diameter of the wire, the wire becomes longer and accordingly each reel has progressively higher rotation rates in order to wind an increasingly longer wire.
The ideal situation would be to provide direct transfer of the wire from one reel to the next.
The essential requirement for correct operation is that the rotation rate of the individual reels must strictly match the tension variations of the wire that leaves the respective dies, adapting the rate to the elongations that the wire undergoes in passing through the various dies.
In practice, this theoretically ideal condition is difficult to achieve because it is difficult to precisely control the rotation rates of the individual motors that drive the reels and because of the variables that come into play during processing, such as die wear etcetera.
In order to obviate these drawbacks, wiredrawing machines have been used which accumulate wire between one reel and the next; more recently, machines provided with a dancer roll or bend detector have been used.
The latter are movable elements onto which the wire being processed is guided. Any variations in the elongation of the wire cause angular or straight-line movements of these elements which, by means of position transducers such as inductive transducers (potentiometers, encoders, etcetera) control and correct the speed of the corresponding motor, thus maintaining uninterrupted processing.
The above described conventional solution, however, has the drawback that they bend the wire around the movable elements, with bending which is sometimes too extreme to obtain optimum metallurgical characteristics of the wire.
Drawbacks also arise due to difficulties in constructing wiredrawing machines that adopt these solutions, in addition to difficulties in threading the machines.
Another solution that has been adopted recently in order to produce wiredrawing machines whose operation is as close as possible to the above described optimum condition entails controlling the speed of the Individual reels, detecting wire tension variations both in the portion between the die and the reel and between the reel and the next die.
The position of the reels and of the dies is fixed; accordingly, since there are no moving elements, control of the speed of the motors of the reels produces variable tensions on the wire in the portion between the reel and the next die.